This invention relates to silicon nitride based material and, more particularly, to silicon nitride/boron nitride composites that have enhanced fracture toughness and high-strength.
Silicon nitride has been identified as a potential candidate for high-temperature structural applications, such as heat engines. Boron nitride additions to silicon nitride have been shown to enhance several properties of silicon nitride. Specifically, high levels of boron nitride additions (10 percent or greater) can enhance such properties as machinability without diamond tooling, thermal shock resistance and the specification of dielectric properties. Although silicon nitride typically exhibits better fracture toughness than many other monolithic ceramics, such as silicon carbide and aluminum oxides, means for further enhancing the inherent fracture toughness of silicon nitride have not previously been developed. It is known, however, that some properties of monolithic ceramics have been improved by incorporating a second phase dispersion, for example, fine monoclinic ZrO.sub.2, BN and Mo particles in Al.sub.2 O.sub.3 or SiC particles in Si.sub.3 N.sub.4.
These aspects of the prior art are discussed in more detail in U.S. Pat. No. 3,813,252 to Leubas, U.S. Pat. No. 3,833,389 to Komeya et al and K. S. Maziyazni and R. Ruh, "High/Low Modulus Si.sub.3 N.sub.4 -BN Composite for Improved Electrical and Thermal Shock Behavior," J. Am. Ceram. Soc., 64, 415-419 (1981).
However, while the previously described Si.sub.3 N.sub.4 /BN composites prepared by hot pressing techniques appeared to have good thermal shock resistance, the composites demonstrated inconsistent densities and the relative theoretical densities of the composites were found to decrease with increasing boron nitride content. Consequently, the room temperature strengths were significantly lower for compositions with BN additions. The hot-pressed composites were also visually unattractive i.e., were found to have varying shades of grayish-white to tan color.